Insufficient viable lung tissue

The average basal oxygen requirement in the resting healthy state is 250 ml/min. At a respiratory quotient of 0.8, which is typical for a Western diet, the average basal production of CO2 in the resting state is 200 ml. Basal oxygen demands may more than double in the presence of fever or infection, and CO 2 production may also increase under these circumstances.

In simple terms, if the lung is acutely unable to cope with these demands, the patient becomes hypoxemic and/or hypercapnic. As the treatment of acute respiratory distress syndrome is basically supportive, in some cases standard mechanical ventilation may not be enough for these increased demands.

G1§ttiQ.0.Q.i eLM: (19.8.7.) postulated that the injured lung could be thought of as comprised of three zones:

1. an area of normal lung (zone H, healthy);

2. an area where the lung function is impaired, but may be improved by therapeutic maneuvers (zone R, recruitable);

3. an area which at present contributes no useful gas exchange, despite therapy (zone D, diseased).

Extrapolating this theory, it may be that the amount of viable lung tissue in zone H and that recruited in zone R, in combination with the extra oxygen demand and CO 2 production due to disease, are insufficient to compensate for zone D. In this case, in order to provide the patient with sufficient PaO2 and to remove an adequate amount of CO2, a non-ventilatory support (extracorporeal respiratory support) must be used.

These techniques do not replace mechanical ventilation but are complementary to it. The technique of extracorporeal CO 2 removal (ECCO2R) is a half-way house, allowing the amount of respiratory support given by a mechanical ventilator to be drastically reduced. This in turn allows airway (distending) pressures applied to the lung to be reduced, as well as reducing PEEP and inspired oxygen concentration.

In comparison, extracorporeal membrane oxygenation (ECMO) transfers a larger amount of oxygen but requires a greater proportion of the cardiac output. Thus the lung is 'rested', and the O2 demands and CO2 removal requirements are met during the bridging period while the lung heals or, in a limited number of cases, until lung transplantation.

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